Braking problem - malfunctioning ABS

travisjb

would be helpful to have someone video your car from a low angle when it happens... i'm still going on the theory that it kicks in when one tire picks up enough to cause significant differential wheel spin rates (as it would on ice)... the conditions you describe sound consistent with this theory... thoughts?

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2009 370Z #104
*For Sale*

bullitt5897

I believe this to be a plausable cause. Mine only kicked in at the end of the braking distances. The car would be able to stop from 60-0 in approximately 50ft and then it would just ice mode to a 75-90ft stopping distance. That would also go to explain why on some surfaces the car would be fine but others it would ice mode hmmm... I think you on to something Travis!

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ResIpsa

Another observation that I forgot to mention. My instructor did not seem all that concerned with the problem. The first time I experienced "ice mode" I was scared that he was going to direct me to pit and end my day. But he didn’t.

The second time it happened he told me to pit and had a marshal eyeball the brakes. The marshal took a look and said that the brakes were definitely hot but otherwise fine.

After that session he solo’d me.

So whatever he perceived in the passenger seat must have felt like simple driver error and not a mechanical issue. I imagine he thought it was simple brake lock up even though there was no tire squeal.

Take this for what it’s worth, I am not sure if it adds anything to the story...

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ChrisSlicks

I can reproduce it on a nice smooth sticky surface with the only factor being temperature. Once the brakes are thoroughly heat soaked ice mode is induced under heavy braking, and this is with totally respectable peak rotor temperatures. I can also prevent ice-mode from occurring by artificially cooling the brakes to prevent the heat soak.

All of my testing has been done on hot and heavy auto-x conditions. It typically takes 20-30 minutes of build up under those conditions to reach a point that ice-mode kicks in for the braking zones.

There are other conditions that activate the ice-mode as well such as sudden bumps, sand, marbles etc, but the heat related one has me beat. What's interesting is that during one of these ice-mode events, at the very end of braking once the car gets below about 15-20mph you suddenly get full braking back again. Totally bizarre.

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travisjb

trying to connect the theories... possible that as your brakes overheat, you're getting fade on inside front wheel and that wheel is therefore braking at a slower rate than the rest?

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2009 370Z #104
*For Sale*

ChrisSlicks

I don't think the overall temperatures are high enough to induce pad fade but it is a theory I will investigate.

One thing that doesn't add up with that theory is that the brakes fully return after the car slows down to a slow speed the ABS computer apparently "exits" its ice-mode. There is also no pull or anything else that would indicate more braking for on one side or the other.

There is however a large temperature differential front to rear, so it is possible that the ratio of front braking force vs rear braking force exceeds the ABS computer parameters causing the event.

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ResIpsa

I will also confirm that after an ice mode event the brakes come back at 100% efficiency. If it was purely heat related I would expect some residual loss of braking following an ice mode event.

Also, I can confirm that during ice mode I never felt like the rear end was going to come around on me. Rather, the car just tracked straight ahead. This leads me to believe that whatever our ABS is doing is to create a heavily front biased brake condition. This of course would create a easily controlled straight slide (for anyone who does not know, too much rear brake bias creates a dangerous situation under braking whereby the rear of the car slides out much like a power slide under heavy acceleration).

Also, this "ice mode" problem is not limited to the 370Z. Google the term ice mode and brakes and you will be amazed at how many cars have the exact same problem.

My theory is that this is a result of ABS technology advancing faster than real world practicality. I imagine there is some algorithmic equation in our cars brain that decides it is better to allow the car to track in a straight line at a controllable 25% braking than to lock one wheel up or flip the car around.

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RCZ

I guess its time to adjust our driving style folks...

travisjb

solutions on the table:

- stiffer suspension
- more brake cooling
- switch ABS computers
- less aggressive compounds
- adjust driving style

Miss any ?

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2009 370Z #104
*For Sale*

cossie1600

I only have two autox on the car so far, but ice mode was only apparent during complete panic stop when I have to do a complete stop at the finish line. It was never a real issue when I was running. I have a stock car with 265 and 285 tires

I have experienced ice mode in my 350, C6, RX-8, you just get used to it.

cossie1600

I only have two autox on the car so far, but ice mode was only apparent during complete panic stop when I have to do a complete stop at the finish line. It was never a real issue when I was running. I have a stock car with 265 and 285 tires

I have experienced ice mode in my 350, C6, RX-8, you just get used to it.

ChrisSlicks

This is where it seems to get a little different between us.

I can brake as hard as I want for about 3 runs. After that I get ice-mode under firm straight line braking every time. It doesn't matter how gently I roll on to the brake either. The brakes are hot but not cooked, and the pads have twice the operating temperature range of stock (Mintex Extreme) but are not an aggressive compound at all. The brakes have a 15-minute cool down between runs.

The front ABS actuators pulse away like crazy as if there is lock up yet there is none. I have tried altering the brake bias to the front and to the rear by using a more aggressive pad on one axle at a time. This had no benefit.

Yes, stiffer suspension all around will prevent the weight transfer, and this is on the cards anyway, but the fact that it only happens to me when the brakes are warm makes me think there is something else at play here. Either the ABS computer is getting bad input from one of the wheel speed sensors due to heat expansion or something else in the ABS is acting up when the brake fluid temperatures rise.

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travisjb

chris, we need to construct tests for these theories... to rule out (or in) the heat theory, we need to measure temperature on the calipers and rotors and conduct trials... not sure how to test temp of the fluid though

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2009 370Z #104
*For Sale*

ChrisSlicks

I could measure the temperature of the hard brake line at certain points. In theory after a 10 minute sit the line that wraps the fluid should be close in temperature due to conductive heat transfer.

Okay for caliper temps I can use the temperature stickers and probably a portable laser thermometer (should read high enough). I will get some paint for the rotors and will see if 1400F is enough for the pyrometer, the 1800F unit is more expensive.

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ChrisSlicks

Ok picked up an Infrared thermometer from Grainger, it's good up to 1832F. If anything on my car gets that hot I expect spontaneous combustion!

Extech 42512: Dual Laser InfraRed Thermometer

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